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So, to denote that a function returns no value we use `Unit`.
How should I denote that a parameter takes no value? `Unit` or `Nothing?`?

That’s relevant in a generic context like `Visitor&lt;Result, Data&gt;` where some visitors need no input and hence use `Unit` or `Nothing?` as `Data`.

The `Unit` type has one single possible instance (`Unit`), while `Nothing` has no value instances. With that in mind, `Unit -&gt; A` is a function that doesn't require anything interesting to produce an `A`, while `Nothing -&gt; A` is a function that can never be called because it's not possible to create the argument instance to call it with.

The choice is between `Unit` and `Nothing?`, i.e. an *optional* `Nothing`.
Both have exactly one value: `Unit` has `Unit` and `Nothing?` has `null`.

Sorry, I missed the “?”. In that case, if I understand the intent correctly, I’d recommend Unit because “Nothing?” can only ever be null and so the function is only ever called with null. I find Unit conveys the same intent more clearly.

Yeah I thought so too.

I’m surprised that Kotlin compiler code uses `Nothing?` in that case so I think it’s a topic worth discussing :smile:

<https://github.com/JetBrains/kotlin/blob/0e4e5ac287c3836ca4434c2c6f626b9145c284e5/compiler/fir/tree/gen/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt#L128>

Maybe.
Or maybe because `Unit` means there’s a value (`Unit`) and `null` better represents the *absence* of a value.

I’d personally find Unit to better represent the absence of any meaningful value, but... ¯\_(ツ)_/¯